113
(
7
)
, pp.1749
-
1753 , 2016-02-16 , Proceedings of the National Academy of Sciences of the United States of America (PNAS)

ISSN:0027-8424

Description

Surfaces of ice are covered with thin liquid water layers, called
quasi-liquid layers (QLLs), even below their melting point (0 °C),
which govern a wide variety of phenomena in nature. We recently
found that two types of QLL phases appear that exhibit different
morphologies (droplets and thin layers) [Sazaki G. et al. (2012) Proc
Natl Acad Sci USA 109(4):1052−1055]. However, revealing the
thermodynamic stabilities of QLLs remains a longstanding elusive
problem. Here we show that both types of QLLs are metastable
phases that appear only if the water vapor pressure is higher than
a certain critical supersaturation. We directly visualized the QLLs
on ice crystal surfaces by advanced optical microscopy, which can
detect 0.37-nm-thick elementary steps on ice crystal surfaces. At a
certain fixed temperature, as the water vapor pressure decreased,
thin-layer QLLs first disappeared, and then droplet QLLs vanished
next, although elementary steps of ice crystals were still growing.
These results clearly demonstrate that both types of QLLs are kinetically
formed, not by the melting of ice surfaces, but by the
deposition of supersaturated water vapor on ice surfaces. To our
knowledge, this is the first experimental evidence that supersaturation
of water vapor plays a crucially important role in the formation
of QLLs.